In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
The most common substrate material used today by manufacturers of thin film Cu(In,Ga)Se2 (abbreviated CIGS) solar cells is sodalime glass. Two examples of solar cells with glass substrates are DE-A-100 24 882 and U.S. Pat. No. 5,994,163. A positive effect by the use of sodalime glass is an increased efficiency of the solar cell, due to the diffusion of an alkali metal (primarily sodium) from the glass into the CIGS layer. This fact is known from, e.g., the Thesis by Karin Granath (1999): The Influence of Na on the Growth of Cu(In,Ga)Se2 Layers for Thin Film Solar Cells, Acta Universitatis Upsaliensis, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 491, Uppsala ISBN 91-554-4591-8, hereby incorporated into the present disclosure by this reference. However, the batch-like production of CIGS on glass substrates is expensive and, therefore, it is an advantage to use roll-to-roll production of solar cells, which lowers the production cost. Moreover, there are several technical advantages with flexible solar cells produced by a roll-to-roll process, for instance, the flexible solar cells can be folded or rolled into compact packages and they may be used for making light weight solar cells, which is desirable for portable, spatial and military applications.
Several materials have been tested as substrate materials for flexible CIGS solar cells, including polymers such as polyimide and metals such as molybdenum, aluminium and titanium foils, bearing in mind that they all have to fulfill certain criteria. Thus, the substrate material should be thermally resistant in order to withstand further process steps in the production of thin film flexible CIGS solar cells, and this may include heat treatments at temperatures up to 600° C. under corrosive atmosphere. The flexible metallic substrate should be insulated from the electrical back contact if CIGS modules with integrated series connections are to be produced. Therefore, it is essential that the thermal expansion coefficient (TEC) of the substrate material should be as close as possible to the TEC of the electrical insulating metal oxide layer(s) to avoid thermal cracking or spallation of the insulating metal oxide layer. Common conventional substrate materials for the production of CIGS solar cells are:                Using sodalime glass substrates in batch-like processes;        Depositing a molybdenum back contact material directly onto the metal strip that constitutes the substrate;        Depositing insulating silicon oxide (SiOx or SiO2) layers onto metal strips in batch type deposition processes.        
One example of known solar cells are disclosed in Thin Solid Films 403-404 (2002) 384-389 by K. Herz et al.: “Dielectric barriers for flexible CIGS solar modules”, hereby incorporated into the present disclosure by this reference. According to this article, excellent electrical insulation for the preparation of CIGS solar modules was obtained on metal substrates by using SiOx and/or Al2O3 barrier layers. However, due to the lack of sodium, the voltage produced by the solar cell was inferior.
A further example of known solar cells making use of stainless steel substrates are disclosed in Solar Energy Materials & Solar Cells 75 (2003) 65-71 by Takuya Satoh et al.: “Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers”, hereby incorporated into the present disclosure by this reference. However, according to this article, the CIGS solar cells on the stainless steel decreased open-circuit voltage compared with that on the soda-lime glass substrates.
Moreover, in WO 03/007386 (hereby incorporated into the present disclosure by this reference) a thin-film solar cell is described. It comprises a flexible metallic substrate having a first surface and a second surface. A back metal contact layer is deposited on the first surface of the flexible metallic substrate. A semiconductor absorber layer is deposited on the back metal contact. A photoactive film deposited on the semiconductor absorber layer forms a heterojunction structure and a grid contact deposited on the heterojunction structure. The flexible metal substrate can be constructed of either aluminum or stainless steel. Furthermore, a method of constructing a solar cell is disclosed. This method comprises providing an aluminum substrate, depositing a semiconductor absorber layer on the aluminum substrate, and insulating the aluminum substrate from the semiconductor absorber layer to inhibit reaction between the aluminum substrate and the semiconductor absorber layer. Although this known solar cell works satisfactorily, it does not attain the open-voltage level of a solar cell with a soda-lime glass substrate because of the lack of sodium doping.
Thus, all these conventional methods have their respective disadvantages. All processes based on batch-type production will always increase the cost and it is therefore essential that the production will be on a roll-to-roll process to decrease the cost.
Hence, when using sodalime glass, it is impossible to produce flexible CIGS, and the batch-type process is expensive. Further, the deposition of Mo back contact directly onto the flexible metal strip substrate will limit the production of CIGS modules with integrated series connections. Furthermore, the SiOx or SiO2 insulating layers have a too low TEC, which may lead to the formation of cracks and pinholes during the following process steps. Moreover, by not adding an alkali metal in the SiO2 layer, it (primarily sodium) has to be added in a later production step if higher efficiency CIGS is to be produced. The addition of one or more process steps in a production line is always associated with extra costs.